A typical deep-running, lineshaft turbine pump of the prior art is shown in FIGS. 1 and 2. The turbine pump 10 consists of a driver 12, such as an electric motor, a lineshaft 14 driven by the driver 12, a plurality of impellers 16 driven by the lineshaft 14, a column pipe 18 enclosing the lineshaft 14, and a bowl 20 above the impellers 14. The lineshaft 14 runs on a plurality of bearings 22 distributed at intervals along the lineshaft 14. Fluid is brought into the pump 10 through a suction case 24 and is discharged from the pump 10 through a discharge pipe 26. A typical environment in which the turbine pump 10 operates is shown in FIG. 2. The pump 10 is inserted in a well W drilled into the surface S of the earth. A body of fluid F, such as oil or water, is deep in the earth, perhaps several hundreds of feet. The pump 10 is positioned such that the suction case 24 is within the body of fluid F, with the discharge case above the surface S of the earth. Power from the driver 12 is distributed to the impellers 16 by the lineshaft 14, producing suction within the suction case 24, drawing fluid F into the suction case 24. Fluid is forced by the impellers 16 into the bowl 20, and from the bowl 20 into the column pipe 18. Fluid travels up the column pipe 18 to the discharge pipe 26, where it is pumped out. Deepwell lineshaft turbine pumps such as the pump 10 need a device to keep the pump from running backwards if the pump stops due to power failure or normal shutdown. When a pump is shut off, fluid runs back down the column pipe 18 at a very high speed. The force backflow hits and runs the impellers 16 spinning the pump backwards. The centrifugal force developed during backspin can tear the windings out of the rotor of the electric motor which may be used for the driver 12. If a gear drive is used instead, the force will damage the engine that drives it. In addition, pumps 10 are of two types: oil lubricated and product lubricated. In the product lubricated type, the bearings 22 are actually lubricated by the fluid being pumped by the pump. If the pump shuts off, the fluid level drops down the column pipe 18 leaving the bearings 22 running dry, burning the rubber and scoring the shaft 14. In the past, this problem has been addressed by non-reverse ratchets or sprags attached to the driver 12 to prevent the driver 12 from running backward, or by footvalves within the suction case 24. However, such devices can fail during reverse spin because of the horsepower generated by the impellers 16. Furthermore, the pressure developed on footvalves may cause the suction case 24 to fail. Also, these devices stop flow abruptly so that backflow still puts force on the impellers and inflicts torque on the lineshaft. There is a need for an anti-surge valve for such deep-running lineshaft pumps which will solve the backflow problem and avoid the problems of non-reverse ratchets or footvalves.